Ultrasonic spray assisted Mist-CVD method for high-quality crystalline and amorphous oxide semiconductors growth
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1113-F05-09
Ultrasonic spray assisted Mist-CVD method for high-quality crystalline and amorphous oxide semiconductors growth Hiroyuki Nishinaka, Yudai Kamada, Keiji Kiba, Naoki Kameyama, and Shizuo Fujita Dept. Electronic Science and Engineering, Kyoto University, Katsura, Kyoto, 615-8520, Japan
ABSTRACT The growth of high-quality crystalline ZnO thin films on ZnO bulk substrates and of amorphous In-Ga-O and Ga-Al-O thin films has been demonstrated by using the solution-based cost-effective and environmental friendly ultrasonic spray assisted mist-CVD method. The homoepitaxial ZnO thin films with atomically flat surfaces were successfully grown on Zn-polar ZnO substrates via a step-flow growth mode, in spite of different miscut angles of the substrate, at the furnace temperature of 1000oC. The compositions and optical absorption edges of the amorphous In-Ga-O and Ga-Al-O thin films were controlled by means of the concentration ratios of [In]/([In]+[Ga]) and [Al]/([Al]+[Ga]) in the starting solutions. INTRODUCTION Oxide semiconductors have attracted great attentions for optical and electrical devices, such as light emitting diodes [1], lasers [2], transparent conductors [3], and thin film transistors [4]. Structures of oxide semiconductors proposed as these devices include not only single crystals [1,2] but also poly crystals [3] and even amorphous materials [4]. Oxide semiconductors have great advantages over other semiconductor materials, such as Si, GaAs, and GaN, against operation and handling in air atmosphere, because oxygen is not an impurity element but one of the compound elements. Therefore, oxide semiconductors even possess possibility to be grown in the atmospheres of air, water, and other solvents. This encourages the synthesis of oxide semiconductors by inexpensive and simple methods of using the solution processes. Evolution of an inexpensive, safe, and environmental-friendly ultrasonic spray assisted mist-chemical vapor deposition (CVD) method has definitely been shown with the growth of high-quality crystalline and amorphous oxide semiconductors [5-9]. In the mist-CVD, we have used liquid solutions of source materials as precursors. They are ultrasonically atomized and the mist particles formed are transferred to the reaction area with a carrier gas. In this way, the mist sources can substitute for vaporized sources of toxic and/or dangerous metalorganics, and therefore the mist-CVD can possess excellent controllability for a source supply even comparable to that in metalorganic-CVD (MOCVD). We have previously reported the potentials of the mist-CVD technique for the step-flow growth of homoepitaxial ZnO thin films at the initial stage [5], the high quality heteroepitaxial α-Ga2O3 thin films on sapphire (α-Al2O3) [6], the fabrication of ZnO nanostructures [7], and the low-temperature growth of ZnO on plastic substrates [8]. In this paper, we report the following advancements in the mist-CVD technique. First, we show that the step-flow growth of homoepitaxial ZnO films continues to thicker films, fo
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